AppEngine Expando model implemented in SQLAlchemy

sqlalchemy_expando.py

"""
Usage:

class MyTable(ExpandoModel):
    __tablename__ = 'my_table'
    id = Column(types.Integer, primary_key=True)

>>> t = MyTable(id=1)
>>> Session.add(t)
>>> Session.commit()
>>> t.random_property = 42
>>> t.foo = "bar"
>>> Session.commit()
>>> t = Session.query(MyTable).get(1)
>>> t.foo, t.random_property
('bar', 42)

"""

from sqlalchemy import MetaData, types, Column
from sqlalchemy.ext.declarative import declarative_base

metadata = MetaData()

class _ExpandoBase(object):
    """
    Based on:
        https://github.com/shazow/everything/blob/master/idea/arbitrarily-structured-data-in-rdbms.md
        http://code.google.com/appengine/docs/python/datastore/expandoclass.html
    """
    _dynamic_properties = Column(types.PickleType(mutable=False))

    def __getattribute__(self, key):
        # __getattribute__ is necessary to avoid infinite recursion on setattr.
        if not key.startswith('_'):
            dynamic_properties = self._dynamic_properties
            if dynamic_properties is not None and key in dynamic_properties:
                return self.__getattr__(key)
        return super(_ExpandoBase, self).__getattribute__(key)

    def __getattr__(self, key):
        # _dynamic_properties dereferencing is necessary to avoid unnecessary __getattribute__ lookups.
        _dynamic_properties = self._dynamic_properties

        if _dynamic_properties is not None and key in _dynamic_properties:
            return _dynamic_properties[key]
        return getattr(super(_ExpandoBase, self), key)

    def __setattr__(self, key, value):
        if key.startswith('_') or key in self.__table__.c:
            return super(_ExpandoBase, self).__setattr__(key, value)

        _dynamic_properties = self._dynamic_properties
        if _dynamic_properties is None:
            _dynamic_properties = {}
        else:
            _dynamic_properties = _dynamic_properties.copy()

        _dynamic_properties[key] = value

        # Pickles aren't mutable, so we shove a fresh instance of the variable in
        self._dynamic_properties = _dynamic_properties

    def __delattr__(self, key):
        _dynamic_properties = self._dynamic_properties

        if _dynamic_properties and key in _dynamic_properties:
            _dynamic_properties = _dynamic_properties.copy()

            del _dynamic_properties[key]

            # Pickles aren't mutable, so we shove a fresh instance of the variable in
            self._dynamic_properties = _dynamic_properties

        return delattr(super(_ExpandoBase, self), key)

ExpandoModel = declarative_base(metadata=metadata, cls=_ExpandoBase)